Ignore TODOs in analyzer_testing package.

Some time ago, the analyzer team decided to adopt the convention of
using `TODO` comments to document long term issues that should persist
in the codebase, and `FIXME` comments to document short term issues
that need immediate attention.

Accordingly, it makes sense to suppress `TODO` comments from being
surfaced to the IDE "problems" view (since there can be a lot of them,
and they're not immediately actionable).  This makes VSCode's
"problems" view much more usable in "tree" mode.

Previous CLs that have made this change in other packages:
- https://dart-review.googlesource.com/c/sdk/+/295662
- https://dart-review.googlesource.com/c/sdk/+/325121
- https://dart-review.googlesource.com/c/sdk/+/358980
- https://dart-review.googlesource.com/c/sdk/+/410060

Change-Id: Icaa7351b57873f1015014792741ea919ecdf55e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/440546
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Auto-Submit: Paul Berry <[email protected]>

Author: stereotype441

pkg/analyzer_testing/analysis_options.yaml

@@ -3,6 +3,11 @@ include: package:lints/recommended.yaml
 analyzer:
   exclude:
     - lib/mock_packages/package_content/**
+  errors:
+    # Ignoring this for all developers means developers don't need to ignore
+    # TODOs in their IDE settings.
+    todo: ignore
+
   language:
     strict-casts: true
     strict-inference: true